docs: def form_invalid(self, post_form, attachment_formset, **kwargs):
          """ Processes invalid forms.
  
          Called if one of the forms is invalid. Re-renders the context data with the data-filled
          forms and errors.
  
          """
          if (
              attachment_formset and
              not attachment_formset.is_valid() and
              len(attachment_formset.errors)
          ):
              messages.error(self.request, self.attachment_formset_general_error_message)
  
          return self.render_to_response(
              self.get_context_data(
                  post_form=post_form, attachment_formset=attachment_formset, **kwargs
              ),
          )